15,252 research outputs found
Control of equilibrium pressure-temperature conditions in cryogenic storage
Metered vent controls the pressure within a liquid hydrogen tank. Vent size is chosen to permit a gas flow which corresponds to the boil-off rate necessary to maintain the desired bulk temperature of the cryogen
An environmental assessment of the John Pennekamp Coral Reef State Park and the Key Largo Coral Reef Marine Sanctuary (Unpublished 1983 Report)
The Pennekamp Coral Reef State Park was established in 1960 and the Key Largo National Marine Sanctuary in 1975. Field studies, funded by NOAA, were conducted in 1980 - 1981 to
determine the state of the coral reefs and surrounding areas in relation to changing environmental conditions and resource management that had occurred over the intervening
years. Ten reef sites within the Sanctuary and seven shallow grass and hardbottom sites within the Park were chosen for qualitative and quantitative studies. At each site, three parallel transects not less than 400 m long were run perpendicular to the reef or shore, each 300 m
apart. Observations, data collecting and sampling were done by two teams of divers. Approximately 75 percent of the bottom within the 18-m isobath was covered by marine
grasses, predominantly turtle grass. The general health of the seagrasses appeared good but a few areas showed signs of stress. The inner hardbottom of the Park was studied at the two entrances to Largo Sound. Though at the time of the study the North Channel hardbottom was subjected to only moderate boat traffic, marked changes had taken place over the past years, the most obvious of which was the loss of the extensive beds of Sargassum weed, one of the most extensive beds of this alga in the Keys. Only at this site was the green alga Enteromorpha encountered. This alga, often considered a pollution indicator, may denote the effects of shore run off. The hardbottom at South Channel and the surrounding grass beds showed signs of stress. This area bears the heaviest boat traffic within the Park waters causing continuous turbidity from boat wakes with resulting siltation. The offshore hardbottom and rubble areas in
the Sanctuary appeared to be in good health and showed no visible indications of deterioration. Damage by boat groundings and anchors was negligible in the areas surveyed. The outer reefs in general appear to be healthy. Corals have a surprising resiliency to detrimental factors and, when conditions again become favorable, recover quickly from even severe damage. It is, therefore, a cause for concern that Grecian Rocks, which sits somewhat inshore of the outer reef line, has yet to recover from die-off in 1978. The slow recovery, if occurring, may be due to the lower quality of the inshore waters. The patch reefs, more adapted to inshore waters, do not show obvious stress signs, at least those surveyed in this study. It is apparent
that water quality was changing in the keys. Water clarity over much of the reef tract was observed to be much reduced from former years and undoubtedly plays an important part in the stresses seen today over the Sanctuary and Park. (PDF contains 119 pages
Parametric, nonparametric and parametric modelling of a chaotic circuit time series
The determination of a differential equation underlying a measured time
series is a frequently arising task in nonlinear time series analysis. In the
validation of a proposed model one often faces the dilemma that it is hard to
decide whether possible discrepancies between the time series and model output
are caused by an inappropriate model or by bad estimates of parameters in a
correct type of model, or both. We propose a combination of parametric
modelling based on Bock's multiple shooting algorithm and nonparametric
modelling based on optimal transformations as a strategy to test proposed
models and if rejected suggest and test new ones. We exemplify this strategy on
an experimental time series from a chaotic circuit where we obtain an extremely
accurate reconstruction of the observed attractor.Comment: 19 pages, 8 Fig
The identification of continuous, spatiotemporal systems
We present a method for the identification of continuous, spatiotemporal
dynamics from experimental data. We use a model in the form of a partial
differential equation and formulate an optimization problem for its estimation
from data. The solution is found as a multivariate nonlinear regression problem
using the ACE-algorithm. The procedure is successfully applied to data,
obtained by simulation of the Swift-Hohenberg equation. There are no
restrictions on the dimensionality of the investigated system, allowing for the
analysis of high-dimensional chaotic as well as transient dynamics. The demands
on the experimental data are discussed as well as the sensitivity of the method
towards noise
Vortex-glass transition in superconducting Nb/Cu superlattices
Nb/Cu superconducting superlattices have been fabricated by dc magnetron
sputtering. This system shows a vortex glass transition with critical exponents
similar to high temperatures superconductors exponents. The transition
dymensionality is governed by the superconducting coupling regime. The vortex
glass transition shows a pure two dimensional behavior in decoupled
superlattices and a quasi-two dimensional behavior in the superlattice coupling
regime.Comment: 9 pages, 3 figure
Optical-fiber source of polarization-entangled photon pairs in the 1550nm telecom band
We present a fiber based source of polarization-entangled photon pairs that
is well suited for quantum communication applications in the 1550nm band of
standard fiber-optic telecommunications. Polarization entanglement is created
by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed
orthogonally-polarized pump pulses and subsequently removing the time
distinguishability by passing the parametrically scattered signal-idler photon
pairs through a piece of birefringent fiber. Coincidence detection of the
signal-idler photons yields biphoton interference with visibility greater than
90%, while no interference is observed in direct detection of either the signal
or the idler photons. All four Bell states can be prepared with our setup and
we demonstrate violations of CHSH form of Bell's inequalities by up to 10
standard deviations of measurement uncertainty.Comment: 12 pages, 4 figures, to be submitted to Phys. Rev. Lett. See also
paper QTuB4 in QELS'03 Technical Digest (OSA, Washington, D.C., 2003). This
is a more complete versio
Analysis of SPDEs Arising in Path Sampling Part I: The Gaussian Case
In many applications it is important to be able to sample paths of SDEs
conditional on observations of various kinds. This paper studies SPDEs which
solve such sampling problems. The SPDE may be viewed as an infinite dimensional
analogue of the Langevin SDE used in finite dimensional sampling. Here the
theory is developed for conditioned Gaussian processes for which the resulting
SPDE is linear. Applications include the Kalman-Bucy filter/smoother. A
companion paper studies the nonlinear case, building on the linear analysis
provided here
Universal Behavior of the Resistance Noise across the Metal-Insulator Transition in Silicon Inversion Layers
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional (2D) electron system in the vicinity of the metal-insulator
transition occurs in all Si inversion layers. The size of the metallic glass
phase, which separates the 2D metal and the (glassy) insulator, depends
strongly on disorder, becoming extremely small in high-mobility samples. The
behavior of the second spectrum, an important fourth-order noise statistic,
indicates the presence of long-range correlations between fluctuators in the
glassy phase, consistent with the hierarchical picture of glassy dynamics.Comment: revtex4; 4+ pages, 5 figure
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